1. Field of the Invention
The present invention relates to a mold casting process and a mold casting apparatus used for carrying out the process, as well as a method for producing mechanical parts by application of the mold casting process.
2. Description of the Prior Art
There is conventionally known a mold casting process wherein a temperature gradient is applied to a mold to provide a directional solidification, but timing for releasing a casting from the mold is not considered in any way (see Japanese Utility Model Application Laid-open No. 82746/86).
When a cast product is obtained by a casting process using a mold in order to improve the productivity thereof, the following problems are encountered: Due to a high heat transfer coefficient of the mold and the form of the product, the solidification and shrinkage of the cast product is partially greatly accelerated, so that a portion of the product is restrained by the mold, resulting in thermal cracking of the product and damage such as deformation and wearing of the mold.
To provide a product free from casting defects such as cavities, it is necessary to take corresponding measures, but no special measures have been taken in the prior art.
In achieving a product including a first formed portion of a harder structure and a second formed portion of a softer structure in a casting process using a mold, a procedure used in the prior art is to rapidly cool a first formed portion shaping region of the mold with cooling water and to prevent rapid cooling of a second formed portion shaping region of the mold by a block formed of a material such as a shell sand.
The prior art process is accompanied by the following problem: Thermal insulation between the first and second formed portions is not taken into account positively and for this reason, heat transfer take place therebetween, and the manner of such heat transfer is not even. Thus, the structures of the both formed portions are widely different from the intended structure.
With a cast product having a thinner portion and a thicker portion integral with the thinner portion, there is a problem that the cooling rates for both portions are different from each other and hence, releasing a resulting product from a mold at a timing suitable for the thinner portion results in that the thicker portion cannot have a sufficient shape retainability at the time of release, whereas releasing the resulting product at a timing suitable for the thicker portion leads to the possibility of producing thermal cracking in the thinner portion.
Further, in producing a mechanical part blank in a casting process using a mold, it is necessary to correct its shape when a deformation, a bend or the like are produced in the resulting mechanical part blank released from the mold. However, the mechanical part blank after being cooled has a small ductility and hence, a large-sized shape correcting or setting device having a higher pressing force must be provided, resulting in an increase in cost of equipment and in addition, a cracking or the like may be produced, resulting in a defective product.
Yet further, in efficiently producing a high strength cast product having a fine structure through a rapid solidification of a molten metal utilizing a high heat transfer coefficient of a mold, it is required to increase the pouring rate in order to prevent a failure of running of the molten metal. However, increasing the pouring rate only produces casting defects such as cavities and pin holes in the resulting product, because the molten metal is liable to include slag and gas thereinto. In addition, even if a slag removing portion is provided in a molten metal passage communicating with a cavity, a slag removing effect is less achieved, because the molten metal within the slag removing portion may be rapidly solidified to form a solidified layer.
There is also known a mold comprising a convex shaping portion to form a recess in a resulting product, and in such conventional known mold, its body and convex shaping portion are integrally formed of the same material (see Japanese Patent Application Laid-open No. 8382/80).
The aforesaid convex shaping portion may be worn by the flow of molten metal or damaged due to an adhesion force of the cast product attendant upon the solidification and shrinkage thereof. For this reason, if the mold body and the convex shaping portion are integrally formed as described above, a repairing operation on a large scale must be carried out for providing a padding by welding, a machine working or the like to the mold body. Such repairing operation is very troublesome and brings about a reduction in production efficiency.
Moreover, to prevent the trapping of gas into a molten metal, it is a conventional practice to provide a vent hole opened into a cavity in a mold, or to provide a gas venting slit in a split face of a mold.
However, with the above mold, even though gas in the cavity can be forced out and removed by the molten metal before pouring, a gas venting effect is poor after pouring because the molten metal enters and is solidified in the vent hole or slit. This results in that gas produced in the cavity from the molten metal after pouring cannot be sufficiently removed.